Return the value of DeltaT for the given Julian date

**Namespace:** ASCOM.Astrometry**Assembly:** ASCOM.Astrometry (in ASCOM.Astrometry.dll) Version: 6.0.0.0 (6.3.0.2839)

Syntax public static double DeltaT(
double Tjd
)

Public Shared Function DeltaT (
Tjd As Double
) As Double

public:
static double DeltaT(
double Tjd
)

#### Parameters

- Tjd
- Type: SystemDouble

Julian date for which the delta T value is required

#### Return Value

Type:

DoubleDouble value of DeltaT (seconds)

Remarks Valid between the years 1650 and 2050. Based on the polynomial given at http://sunearth.gsfc.nasa.gov/eclipse/SEcat5/deltatpoly.html as retrtieved on 11-Jan-2009.

The Astronomical Almanac table is corrected by adding the expression: -0.000091 (ndot + 26)(year-1955)^2 seconds
to entries prior to 1955 (AA page K8), where ndot is the secular tidal term in the mean motion of the Moon.

Entries after 1955 are referred to atomic time standards and are not affected by errors in Lunar or planetary theory.

See also: Morrison, L. and Stephenson, F. R., "Historical Values of the Earth's Clock Error ΔT and the Calculation of Eclipses", J. Hist. Astron., Vol. 35 Part 3, August 2004, No. 120, pp 327-336 (2004).
Note, Stephenson and Morrison's table starts at the year 1630.The actual accuracy decreases rapidly prior to 1780.

See Also